The human epidermal growth factor receptor (HER) family comprises four distinct receptor tyrosine kinases referred to as HER1 (or erbB1), HER2 (or erbB2), HER3 (or erbB3), and HER4 (or erbB4). HER1 is also commonly referred to as epidermal growth factor receptor (EGFR). With the exception of HER3, these receptors have phospho-acceptor target specific intrinsic protein tyrosine kinase activities. Members of the HER family are expressed in most epithelial cells as well as in a number of different tumor cell types. For example, receptors of the HER family are expressed in tumor cells of epithelial origin, and of mesenchymal origin. Moreover, HER receptor tyrosine kinases are involved in cell proliferation and angiogenesis, which are associated with diseases such as cancer. For example, EGFR is frequently over-expressed or aberrantly activated in breast cancers, liver cancers, kidney cancers, leukemia, bronchial cancers, pancreatic cancers and gastrointestinal cancers such as colon, rectal or stomach cancers. High levels of the EGF receptor also correlate with poor prognosis and response to treatment (Wright et al., 1992, Br. J. Cancer 65:118-121). Thus, disruption of signal transduction from and to these kinases would have an anti-proliferative, and as such, therapeutic effect upon a number of cancer and tumor cell types.
The enzymatic activity of receptor tyrosine kinases can be stimulated by over-expression and/or by ligand-mediated dimerization (Heldin, 1995, Cell 80:213-223). Activation of receptor homodimers and heterodimers results in phosphorylation of tyrosine residues on the receptors, which in turn phosphorylate tyrosine residues of other molecules, including intracellular proteins. (Ullrich et al., 1990, Cell 61:203-212). This is followed by the activation of intracellular signaling pathways such as those involving the mitogen-activated protein kinase (MAP kinase) (Dhillon et al., 2007, Oncogene 26: 3279-3290) and the phosphatidylinositol 3-kinase (PI3 kinase). While activation of these pathways has been shown to increase cell proliferation and inhibit apoptosis, inhibition of signaling mediated by HER family members by either small molecule inhibitors or monoclonal antibodies has been shown to inhibit cell proliferation and promote apoptosis (Prenzel et al., 2001, Endocr. Relat. Cancer 8: 11-31)
Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a 22 kDa, O-glycosylated protein (Higahiyama et al., 1992, J Biol Chem 267: 6205-6212). In its mature form, HB-EGF binds to and activates the EGF receptor and HER4 (Elenius et al., 1997, EMBO 16:1268-1278). HB-EGF is the key mediator of G-protein coupled receptor (GPCR) induced cell proliferation via a process called triple-membrane passing signaling (IMPS) (Prenzel et al., 1999, Nature 402:884-888, review in Fischer et al. 2003, Biochem. Soc. Trans. 31:1203-1208). It has been shown that HB-EGF promotes cellular proliferation as well as angiogenesis (Zushi et al., 1997, Int J Cancer 73:917-923; Abramovitch et al., 1998, FEBS letters 425:441-447). HB-EGF also has been demonstrated to play a key role in a number of cancers, i.e., it has been linked to the aggressive behavior of ovarian tumors (Tanaka at al., 2005, Clin. Cancer Res. 11:4783-4792). Moreover, HB-EGF is essential for xenograft tumor formation by ovarian cancer cell lines. Over-expression of HB-EGF (wild type or a secreted form) accelerates tumor formation in SKOV3 and RMG-1 cells. Knockdown of endogenous HB-EGF using siRNA, yet, abolished or delayed tumor formation by SKOV3 and RMG-1 cells. Miyamoto, 2004, Cancer Res. 64:5720. As suggested by the above evidence, inhibition of HB-EGF expression or activity may inhibit tumor formation.
Similarly, HB-EGF is a marker of poor prognosis in some cancers, including human bladder cancers (Thogersen et al., 2001, Cancer Res. 61:6227-6233). In vitro studies indicate that human EJ bladder cells that were engineered to express HB-EGF (wild type, soluble or non-cleavable) exhibit an increase in growth, anchorage independent growth, and production of VEGF, and enhanced migration. When these HB-EGF-expressing EJ bladder cells were transplanted into nude mice, an increase in tumor formation, size and density of blood vessels was observed in those tumors. (Ongusaha, 2004, Cancer Res. 64:5283-5290).